java - 当存在module-info.java时，findResource (“”)返回null，为什么呢？

我正在调试为什么在Spring Boot应用程序中存在module-info.java时，spring-orm在启动期间会引发异常。这是异常(exception):

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is java.lang.NoClassDefFoundError: javax/transaction/UserTransaction
    at [e-mail]/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1699) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:573) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1089) ~[spring-context-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:859) ~[spring-context-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.4.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.SpringApplication.refresh(SpringApplication.java:762) [spring-boot-2.0.4.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:398) [spring-boot-2.0.4.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.SpringApplication.run(SpringApplication.java:330) [spring-boot-2.0.4.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.SpringApplication.run(SpringApplication.java:1258) [spring-boot-2.0.4.RELEASE.jar:na]
    at [e-mail]/org.springframework.boot.SpringApplication.run(SpringApplication.java:1246) [spring-boot-2.0.4.RELEASE.jar:na]
    at tech.flexpoint.dashmanserver/tech.flexpoint.dashmanserver.DashmanServerApplication.main(DashmanServerApplication.java:13) [classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na]
    at [e-mail]/org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.0.4.RELEASE.jar:na]
Caused by: java.lang.NoClassDefFoundError: javax/transaction/UserTransaction
    at java.base/java.lang.Class.getDeclaredMethods0(Native Method) ~[na:na]
    at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3119) ~[na:na]
    at java.base/java.lang.Class.privateGetPublicMethods(Class.java:3144) ~[na:na]
    at java.base/java.lang.Class.getMethods(Class.java:1863) ~[na:na]
    at [e-mail]/org.hibernate.service.internal.AbstractServiceRegistryImpl.applyInjections(AbstractServiceRegistryImpl.java:288) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.service.internal.AbstractServiceRegistryImpl.injectDependencies(AbstractServiceRegistryImpl.java:279) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:239) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:210) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.service.internal.SessionFactoryServiceRegistryImpl.getService(SessionFactoryServiceRegistryImpl.java:80) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.internal.SessionFactoryImpl.canAccessTransactionManager(SessionFactoryImpl.java:942) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.internal.SessionFactoryImpl.buildCurrentSessionContext(SessionFactoryImpl.java:953) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:319) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:462) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:892) ~[hibernate-core-5.2.17.Final.jar:na]
    at [e-mail]/org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:57) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:365) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:390) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:377) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.afterPropertiesSet(LocalContainerEntityManagerFactoryBean.java:341) ~[spring-orm-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1758) ~[spring-beans-5.0.8.RELEASE.jar:na]
    at [e-mail]/org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1695) ~[spring-beans-5.0.8.RELEASE.jar:na]
    ... 21 common frames omitted
Caused by: java.lang.ClassNotFoundException: javax.transaction.UserTransaction
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582) ~[na:na]
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:190) ~[na:na]
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:499) ~[na:na]
    ... 42 common frames omitted

我将问题归结为URLClassLoader.findResource("")(如果存在null，则返回module-info.java)，如果没有，则返回"file:/C:/Users/pupeno/Documents/Dashman/code/dashmanserver/target/classes/"。

我创建了引发相同异常的最小示例。要运行它，您需要:

由于此错误修复尚未发布，请从此处克隆并安装Moditect的最新副本:https://github.com/moditect/moditect:https://github.com/moditect/moditect/issues/51

从以下位置克隆演示存储库:https://github.com/dashmantech/demo

使用凭证demo/confi/application.properties设置本地PostgreSQL数据库

首先运行mvn clean package，以便ModiTec创建所有模块

在IntelliJ的最新副本

中打开项目

单击“运行演示”配置文件的播放(适当的运行配置文件中包括.idea目录，参数等)。

我需要findResource("")返回"file:/C:/Users/pupeno/Documents/Dashman/code/dashmanserver/target/classes/"，以便spring-orm可以工作。
findResource("")看起来像这样:

public URL findResource(final String name) {
    /*
     * The same restriction to finding classes applies to resources
     */
    URL url = AccessController.doPrivileged(
        new PrivilegedAction<>() {
            public URL run() {
                return ucp.findResource(name, true);
            }
        }, acc);

    return url != null ? URLClassPath.checkURL(url) : null;
}

因此，我可以看到无需使用模块系统就可以进行一些访问，但是当存在module-infe.java时，Java的模块系统可以阻止这种访问。我的问题是我看不到如何使它起作用，应该导出或打开什么使其起作用？

Spring Boot导致该方法调用的方式是通过RestartClassLoader的子类URLClassLoader，特别是在第124行中，该行在以下位置调用super.findResource(name):

@Override
public URL findResource(String name) {
    final ClassLoaderFile file = this.updatedFiles.getFile(name);
    if (file == null) {
        return super.findResource(name);
    }
    if (file.getKind() == Kind.DELETED) {
        return null;
    }
    return AccessController
            .doPrivileged((PrivilegedAction<URL>) () -> createFileUrl(name, file));
}

所使用的特定RestartClassLoader实例是ClassPathResource的成员，它是通过以下方式定义的:

this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());

在构造函数line 85中。

最后，getDefaultClassLoader()看起来像这样:

/**
 * Return the default ClassLoader to use: typically the thread context
 * ClassLoader, if available; the ClassLoader that loaded the ClassUtils
 * class will be used as fallback.
 * <p>Call this method if you intend to use the thread context ClassLoader
 * in a scenario where you clearly prefer a non-null ClassLoader reference:
 * for example, for class path resource loading (but not necessarily for
 * {@code Class.forName}, which accepts a {@code null} ClassLoader
 * reference as well).
 * @return the default ClassLoader (only {@code null} if even the system
 * ClassLoader isn't accessible)
 * @see Thread#getContextClassLoader()
 * @see ClassLoader#getSystemClassLoader()
 */
@Nullable
public static ClassLoader getDefaultClassLoader() {
    ClassLoader cl = null;
    try {
        cl = Thread.currentThread().getContextClassLoader();
    }
    catch (Throwable ex) {
        // Cannot access thread context ClassLoader - falling back...
    }
    if (cl == null) {
        // No thread context class loader -> use class loader of this class.
        cl = ClassUtils.class.getClassLoader();
        if (cl == null) {
            // getClassLoader() returning null indicates the bootstrap ClassLoader
            try {
                cl = ClassLoader.getSystemClassLoader();
            }
            catch (Throwable ex) {
                // Cannot access system ClassLoader - oh well, maybe the caller can live with null...
            }
        }
    }
    return cl;
}

我的module-info.java包含:

module tech.flexpoint.dashman {
    exports tech.flexpoint.dashman to com.fasterxml.jackson.databind;
    exports tech.flexpoint.dashman.controllers.configurator to javafx.fxml;

    opens tech.flexpoint.dashman to javafx.graphics, jna;
    opens tech.flexpoint.dashman.controllers.common to javafx.fxml;
    opens tech.flexpoint.dashman.controllers.configurator to javafx.fxml;
    opens tech.flexpoint.dashman.models to org.hibernate.validator, tech.flexpoint.dashmancommon, javafx.base;

    opens common;
    opens configurator;
    opens displayer;
    opens winscreensaver;

    requires appdirs;
    requires org.bouncycastle.provider;
    requires com.fasterxml.jackson.core;
    requires com.fasterxml.jackson.databind;
    requires com.fasterxml.jackson.datatype.jdk8;
    requires io.sentry;
    requires jackson.annotations;
    requires java.desktop;
    requires java.sql;
    requires java.validation;
    requires javafx.controls;
    requires javafx.fxml;
    requires javafx.graphics;
    requires javafx.media;
    requires javafx.web;
    requires jna;
    requires jna.platform;
    requires org.apache.commons.lang3;
    requires org.kordamp.ikonli.javafx;
    requires org.kordamp.ikonli.fontawesome5;
    requires spring.core;
    requires spring.retry;
    requires spring.web;
    requires tech.flexpoint.dashmancommon;
}

在IntelliJ中，我启用了以下插件:

Lombok插件

.ginore

PowerShell

VisualVM启动器

ANSI荧光笔

批处理脚本支持

字节码查看器

CMD支持

版权所有

覆盖范围

CSS支持

数据库工具和SQL

Git集成

GitHub

Gradle

Groovy

Heroku集成

HTML工具

HTTP客户端

Java的

l18n

IDE设置同步

Java字节码反编译器

Java EE:EJB，JPA，Servlet

Java流调试器

JavaFX

JUnit

行分类器

Markdown支持

Maven集成

Maven集成扩展

持久性框架支持

属性支持

Smali支持

Spring AOP/@ AspectJ

Spring 批

Spring 启动

Spring 数据

Spring 集成模式

Spring OSGi

Spring Security

Spring支持

Spring Web服务

Spring WebSocket

终端

YAML

最佳答案

我注意到的一件事是，您的应用程序(假设它打包在tech.flexpoint.dashman中)似乎没有以任何方式向Spring开放，这肯定会导致类加载/非法访问失败。

我希望在module-info.java中看到类似这样的内容(取决于您的Spring依赖项):

opens tech.flexpoint.dashman to spring.core, spring.beans, spring.context;

异常(exception)是NoClassDefFoundError，当无法解析在编译时已知的类的类定义时会在运行时抛出，在这种情况下，该接口(interface)是Java Transaction API (JTA)的一部分javax.transaction.UserTransaction。

正如其他人指出的那样，JTA没有与JDK bundle 在一起，需要作为编译依赖项添加。但是，需要加载UserTransaction类定义的类来自spring-boot-autoconfigure工件，该工件负责其自己的依赖项( [e-mail] 🡒 [e-mail] 🡒 [e-mail] )，因此您无需添加JTA作为依赖项。

但是，由于您要将自己的应用程序打包为Java 9模块，因此需要explicitly state its dependencies。 spring-boot-autoconfigure尚不是模块化的Java 9库，因此不会为您(即，传递地)执行此操作。 JTA的自动模块名称为java.transaction，因此您需要在module-info.java中添加需求:

requires java.transaction;

我运行了您的示例，并且从IntelliJ IDEA运行时确实获得了NoClassDefFoundError。 stacktrace指向ClassNotFoundException，它指示类路径问题。由于IDEA在从那里启动应用程序时会计算类路径，因此我想看看在使用spring-boot-maven-plugin运行应用程序时是否可以重现错误。

我将IDEA运行配置复制到spring-boot-maven-plugin配置，如下所示:

<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <configuration>
      <mainClass>tech.flexpoint.demo.DemoApplication</mainClass>
      <jvmArguments>--show-module-resolution --add-opens=java.base/java.lang=spring.core --add-opens=java.base/java.io=tomcat.embed.core --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.rmi/sun.rmi.transport=ALL-UNNAMED</jvmArguments>
      <workingDirectory>${project.basedir}</workingDirectory>
  </configuration>
</plugin>

然后，我调用了mvn spring-boot:run和voila，该应用程序成功启动，没有错误。
我只能得出结论，这是IntelliJ计算出的类路径的问题。